Loading system



J. R. CARSON ET AL LOADING SYSTEM Filed June 4:, 1921.

Patented Dec. 8, 1925.

UNITED STATES PATENT omer..

JOHN R. CARSON, OF HARMON, ALVA B.

CLARK, 0F BROOKLYN, vNEW YORK, AND

JOHN MILLS, OF WYOMING, NEW JERSEY, ASSIGNORS TO AMERICAN'r TELEPHONE .AND TELEGRAPH COMPANY, A CORPORATION OF NEW YORK. y

LOADING SYSTEM.

Application filed June 4,

To all whom t may concern Be it known that we, JOHN R. CARSON, ALva B. CLARK, and JOHN MILLS, residing at Harmon, Brooklyn, and Wyoming, in the countiesof llfestchester, Kings, and Essex and States of New York, New York, and New Jersey, respectively, have invented certain Improvements in Loading Systems, of which the following is a specification.

This invention relates to loaded lines for the transmission of telephonie currents and the like. The object of the invention is to provide a system of loading applicable especially to long lines, which will improve the quality of transmission. l

The Papin-Campbell system of loading, as originally conceived and hitherto practiced, has as its main object the reduction of attenuation, thereby increasing the possible range of the telephonie transmission of speech. \Vith the advent of the etlicient telephone repeater, mere reduction of the attenuation by loading ceased to have its former fundamental im ortance, while at. the same time, the possi le length of lines for telephonic communication was much 1ncreased. 'These considerations made it necessary to give more serious attention to .the question of the quality of the transmisslon and the received speech, because in long lines various factors combine to impair the quality. Three of these factors which require particular attention are, first, equal attenuation for currents of the various frequencies within the telephone range, second, the time of transmission of the s cech impulses, and third, the transient istortion. All three of these factors tending to impair the quality of the received speech are afected injuriously by the system of loading as now practiced.

As to the requirement for equal, attenua'- tion, it will be clear from a moments consideration that the received currents which represent telephonically transmitted speech must be a faithful copy of the corresponding currents which enter the transmission sys'- tem at the sending end, if the system is not to distort the speech'. In practice, we`are concerned only with the currents of those frequencies which go to make up speech, which, for practical purposes. may be taken as lying between 200 and 2500 cycles per second. If these frequencies are transmitted 1821. Serial No. 474,878.

over the line with equal attenuation, the system may be said to be distortionless rom'the steady-state standpoint. In actual systems, however, this requirement alone is insuiicient to insure good quality and as the length of the circuit is increased, it is found that the time of transmission and the transient distortion play increasingly important rles.

YVith respect to the eifect ot' the time of transmission, it has been found that when 'the time is increased, due, for example, to

the lengthening of the line beyond a certain point, reflected currents produce serious eftect upon the qualit of 'the received speech and also serious au itory andpsychological etlects upon the speaker, who hears his own words return to him as an echo. This particular diliculty may be minimized by increasing the losses in the circuit or by reducing the time of transmission. however, must be kept below a certain point to meerh commercial requirements.

Vith respect to the part played by transient distortion, the currents, after arriving at the distant end of the line, require an appreciable time to build up, and under certain conditions never build upto anything remotely resembling the transmitted current in the short interval during which the latter exists. The effect produces, or may produce, serious impairment in the quality and intelligibility of the received speech even Whenthe line is so designed that the steady-state attenuation of all currents Within the speech range is lsubstantially constant, and, therefore, the system is, from the steady-state standpoint, substantially distortionless.

For relatively short lines, the evil effect of the ordinary loading upon these three factors aifecting the quality of transmission may be disregarded because the effects are relatively inappreciable, but as the range of telephonie transmission is increased, a point is reached where the impairment of quality becomes so serious as to materially reduce the commercial eiciency of the circuits.

The present invention proposes to overcome the diiiiculties above described by properly relating the loading tothe length of the line. Generally speaking, the value of the ,inductance for each loading section should bel proportioned inversely with respect to the length of the line. Length of line, in

The losses,

eect produced by the line or the transient distortion of the signal currents.

In the accompanying drawing, Figure 1 illustrates a line to which the present invention is applicable; Fig. 2 is a curve representing the frequency-attenuation characteristic of a loaded line.

In Fig. 1, X represents one terminal of the line and Y another terminal. Loading coils are indicated diagrammatically at 1, Q, 3 and 4. On long lines of the character to which the invention is particularly applicble, amplifiers will be used as indicated at For any given loading of a line there is a frequency above which the currents are attenuated by the loading per se; that is to say, the presence of the loading `coils themselves introduces attenuation. This effect is illustrated in Fig. 2, where the dotted line represents attenuation due to loading alone in an ideal transmission line, and the full line the actual attenuation, taking into account the resistance of the line. The frequency at which the attenuating effect of the loading coils becomes marked is called the critical frequency vand. is denoted on the drawing by fc.

In order that the invention may be delined with sufficient clearness that one skilled in the art may readily practice the same, the manner of proportioning the loading and the principles upon lwhich the invention is based will be set forth. Let L be the total inductance of the line (including loading coils); R the total resistance; C the total capacity,

' and n the number of loading sections. Then the critical frequency fc is given by And the attenuation of the line (except at very low frequencies) by anwb-@Y I N11/m (3) If t2 denote the time required for a current of frequency f toy build up to its proximate steady-state after arriving at the end of the line, it may be shown theoretically that h If fm is the maximum frequency which it is necessary to transmit, then in terms of t, and t2 the foregoing formulae give 1?, 1rt1fo (6) il L (7) L' ==nductance of load coil peglecting the distributed inductance of the ine.

It will be observed that formulae 5-8 completely determine the loading design (inductance and s acing of load coils) in terms of t1, t, and Cpwhich is determined by the length of the circuit and the wire gauge and spacing. Now as stated previously the time of transmission t, and the building up time t2 must not exceed certain experimentally determined values for a prescribed standard of quality. Letting T1 and T2 denote these values, the requirements of the prescribed standard of quality are satisfied if:

Eample 1.

1150 miles #19 Ga. Cable C==0.7x10" farads T1==0.05 T3=0.005 I fm=2300 L (0.05)2

:70.7X10* =36 henrys 3.025 a 2 f =(2300) .525 f., (2.40) (2300) VThe inductance per loading coil should not exceed 41 mh. The spacing of loading coils should not exceed 1.3 miles.

Eample 2.

115 miles #19 Ga. Cable C20/7x105 farads T,:0.005

fnl-:2300

In this circuit, since the capacity is only -IU that-of the preceding example, it is possible to have a total inductance of 360 henrys instead of 36 and still keep t, less than its upper limit 0.05 seconds. Such a value is unnecessary for transmission purposes; instead we may tentatively take Lzt (36) which makes the equivalent diributed inductance 4 times that of the 1150 mile systern.V This gives Then from the design formulae it follows at once that fcgaoao; nge? and the loading coil spacing must not exceed 1.18 miles and the load coil inductance must not exceed 148 mh.l

In the foregoing examples no consideration has been given to the question of the variation of attenuation AA over the speech range. From equation (2) AA maybe writwhich diminishes in general as the cut-otf frequency is increased. Hence, if it required that AA shall not exceed a prescribed value this requirement may set a still higher limit to the cut-oit frequency fc. The appropriate design procedure is to determine the loading design and fc from formulae I, II, III and then calculate AA from IV. If the attenuation variation thus computed falls Within the prescribed limit, the design is satisfactory; otherwise the cut-oli' frequency must be increased and the loading design recalculated from formulae I, II and III. As an example of this appropriate procedure, assume that in system of exam le #1the requirements regarding allowa le .attenuation variation make it necessary to increase the cut-ofi' frequency fc to 6000 cycles per second. The design of the system may then be modied in either of the following 2 ways:

1) If we kee the total inductance constant it follows rom equation (1) that the number of load coils must be increased in the ratio 6000/5520z- 1-09 which means that the spacing must not exceed (1.3) (.92) :1.19

Aceed (fil) (.85):35 mh.

To summarize the foregoing there is dis-- closed a loading system and a method of design thereof with reference to the length of the stem and the resistance and capacity o the Wires, such that transmission over the circuit satisfies preassigned quality requirements as regards (1) variation of attenuation over the telephonie range; (2) echo eii'ects, and (3) transient distortion.

What we claim is:

1. The method of improving the signal transmitting plroperties of a periodically loaded line, W ich consists in reducing the inductance of the line per unit of len th throughout the line as the length of the luxe is increased.

2. The method of improving the signal transmitting roperties `of ay periodically loaded line, wl ich consists in reducing the inductance per unit of length throughout lthe line as the echo effect upon the line inspeed of wave transmission over the line as a whole as the transient distortion of the signals increases.

4. The method of improving the signal transmitting properties of a periodically loaded line, which consists in proportioning the loading of the line in an inverse relation to the length of the line so that the time of transmission of signal waves shall not. exceed a redetermined value sufiiciently small to avoid echo e'ects. u

5. The method of improving the signa-l transmitting properties of a periodically loaded line, which consists in so relating the loading inversely to the length of the transmission line that the time re uired for a signal impulse to build up at tie receivin end shall not exceed the t1me during whic the impulse persists by more than an amount determined to be consistent with echo qualities. v

6. The method of improving the signal transmitting roperties of a periodically loaded/line w ich consists in reducing the loading ofthe line, as compared to that of a line of different length, in inverse proportion to the length, to such a point that the cutoi fr uenc is substantially greater than' the highest requency necessary to transmit speech, whereby the variation in attenuation of all necessary frequencies is within predetermined limits. e

7. In a signal transmitting system, a transmitting line of such length that the time of transmission of a signal impulse becomes a factor effecting signal quality at the receiving end, and loading coils connected at intervals in said line whose inductance and spacing bear a definite relation to the length of the line such that the total time of transmission is reduced below that producing substantial echo effects and transient distortion.

8. A signal transmitting line in which the number of loading sections is equal to or greater than (TH-T1)2 )95 and the inductance of cach section is equal sent respectively the limiting values for the to or less than v Where T1 and T3 repretime of transmission and time of building up and the inductance of each section is equal 2 to or less than gb Where T1 and T, repre sent respectively the limiting values for the time of transmission and time of building up of the signal, consistent with good signal quality, and fm represents the maximum frequency which it is necessary to transmit, tn, is the number of loading sections, and C the capacity of the line, and in which the cut-0E frequency of the line is sufliciently high so that all frequencies of fm and under are transmitted with substantially equal at.- tenuation.

10. A long loaded transmission line comprising loading coils periodically spaced and amplifiers, in which the loading coils are of such inductance and spacing that the cut-off frequency exceeds twice the highest frequency necessary for telephonie transmission of speech.

In testimony whereof, we have signed our names to this specification this 31st day of May, 1921.

JOHN R. CARSON. ALVA B. CLARK. In testimony whereof, I have signed my name to this specification this 1st' day of June, 1921.

JOHN MILLS. 

